Optoelectronic display devices, such as those based on the Electrically Controlled Birefringence (ECB) effect, the Twisted Nematic (TN) effect and Super-Twisted Nematic (STN) effect (S. W. Depp et al., Sci. Am. Vol. 268, p. 40 (1993); T. Scheffer et al., in “Liquid Crystals, Applications and Uses”, Vol. 1. B. Bahadur, Ed., Chapter 10, pp. 231-274 (1990), Worlds Scientific. Singapore; S. Morozumi, ibid., Chapter 7, pp. 171-194), are well known in the art and are of major importance in a large variety of applications.
In these devices (cf. FIG. 1A), typically, a liquid crystalline material is sandwiched in a cell (3) between two light-polarizing means (4) and the liquid crystalline material is used to electrically rotate the polarization direction of polarized light and, thus, the structure functions as a light valve. Polarized light is produced in these devices by a first polarizer, and a second polarizer (also referred to as analyzer) is used to generate visual perception of the electrical switching of the liquid crystalline material.
As is well established in the prior art, a thin layer of well-ordered liquid crystalline material has the ability to change the state of polarization of light that passes through it. As the LC order can be controlled by means of an electrical field, electric switching can modulate the polarization of the transmitted light. Thereto, typically, a cell structure (cf. FIGS. 1A, B), based on spacered transparent carrier plates (3b) and provided with two transparent electrodes (3c), is utilized where in between the thin layer of LC material (3a) is applied. The transparent electrode material can be, for instance, indium tin oxide, but may also consist of a transparent conducting polymer. In the field-off state, or at low voltages below a threshold value (switching state “off”), the molecular organization within the liquid crystal layer is planar and well-controlled by orientation layers (3d) applied on the electrodes and/or by the presence of small amounts of chiral dopants in the liquid crystal mixture. Well-known and widely used effects are: (i) electrically controlled birefringence effect in which the molecules are aligned unidirectionally (parallel alignment at the orientation layers, no chiral dopant); and (ii) twisted nematic or super-twisted nematic effects in which the molecules are rotated over, for instance, 90° (TN) by means of a perpendicular alignment at the orientation layer. In the field-on state (switching state “on”), the LC molecules align themselves along the electrical field lines. In the limit, i.e. at the fully addressed state at higher voltages, the LC material is aligned perfectly perpendicular to the electrode surfaces and behaves isotropically for the light which passes parallel to the field lines. This means that the state of polarization of light that passes the cell remains unaffected. At intermediate voltages, also intermediate states of orientation can be achieved in which the director, being the local direction of the average orientation of the LC molecules, describes a complicated pattern over the thickness of the cell, which can be used to create intermediate scales between the fully-off and the fully-on state.
When these devices comprise only conventional absorbing polarizers, they change upon switching from a light grey to a dark appearance. Colors are generated by using, for example, additional color filters (U.S. Pat. No. 5,099,345) or a series of selective dichroic, filtering polarizers (U.S. Pat. No. 5,122,887).
A common dichroic sheet polarizer usually absorbs at least 50% of the incident light. In a typical TN display, more than 80% of the incident light is absorbed by the polarizers and color filters and, consequently, the brightness of the displays is very limited. The poor light efficiency of these displays, generally, is compensated by rather intense backlighting, which limits the life-time of batteries in devices such as lap-top computers and portable telephones. Moreover, conventional polarizers transfer the absorbed light into heat and, consequently, exhibit excessive heating-up when used in combination with high-intensity light sources. These high-intensity light sources are employed in applications such as projection televisions, and the polarizers limit the life-time of the displays in these applications. Also in reflective-mode devices, the above concerns with respect to efficiency of polarizers and color filters lead to poor visibility.
The use of photoluminescent, for example fluorescent or phosphorescent, matter to overcome these problems has been described. For example. U.S. Pat. No. 3,844,637 discloses the use of fluorescent liquid crystals. Fluorescent material dissolved or dispersed in liquid crystals also has been suggested (U.S. Pat. No. 4,336,980; H. J. Coles et al., Liq. Cryst., Vol. 14, pp. 1039-1045 (1993)). The use of fluorescent particles as a light trap was described in U.S. Pat. No. 4,405,210. U.S. Pat. No. 4,470,666 discloses the use of a partially transmissive fluorescent color filter. U.S. Pat. Nos. 4,113,360, 4,394,068 and 5,018,837, W. Greubel et al. (Elektronik. Vol. 6., pp. 55-56 (1977)) and M. Bechtler et al. (Electronics December 8, pp. 113-116 (1977)) describe the use of a plate of fluorescent synthetic material in displays as a light trap. U.S. Pat. No. 5,608,554 describes the use of a phosphor layer to enhance the luminance and viewing angle of standard liquid crystal display devices. Also, the use of a layer comprising fluorescent species in the three primary colors has been disclosed (French application FR 2 600 451-A1; U.S. Pat. No. 4,678,285). German patent No. DE 2640909 C2 and G. Baur et al. (Appl. Phys. Lett. Vol. 31, pp. 4-6 (1977); W. Greubel et. al., Elektronik, pp. 55-56 (1977)) disclose a display device that comprises an at least partially mirrored polymer plate (cf. FIG. 2, (A)) in which a fluorescent dye is dissolved and that further comprises a means (B) positioned behind the polymer plate (A), from the light-exit direction, that allows transmission of only that part of the light spectrum that is absorbed by the fluorescent plate (A). Furthermore, in the same patent, which is directed towards the use of a fluorescent plate as a light trap in displays, it is suggested that a plate (A) can be used in which form-anisotropic fluorescent molecules are dissolved in a uniform orientation within the polymer plate. The proposed display configurations suffer a number of drawbacks, most of which are directly related to the dimensions of the plate. First, the light collection area of the fluorescent plate must be substantially larger than the area of the electrooptical light valve to provide enough of the necessary radiation. These displays are therefore only suitable for applications where there is enough room for such a plate such as measuring instruments or digital clocks (M. Bechter et. al., Electronics, December 8, pp. 113-116 (1977)). Secondly, according to other publications by the authors of the prior art patent (G. Baur et al., Appl. Phys. Lett., Vol. 31, pp. 4-6 (1977): W. Greubel et. al. Elektronik, pp. 55-56 (1977)) the fluorescent plate needs to be thick (>1 mm) to generate a high absorption of environmental light. According to the authors, the dye concentration in the plate must be low to avoid “self-absorption” and consequently the plate thickness must be high to generate a light absorption close to 100%. The absorption (A) of the incident light on the surface of the plate is:A=1−10ε(λ)ct where ε(λ) is the molar extinction coefficient of the fluorescent dye at wavelength λ, c is the molar concentration of the fluorescent dye and t is the plate thickness. According to publications by the authors of the prior art patent (G. Baur et al., Appl. Phys. lett., Vol. 31, pp. 4-6 (1977); W. Greubel et. al., Elektronik, pp. 55-56 (1977)) typical values for the extinction coefficient of the fluorescent dye and the dye concentration are, respectively, 104 L/mole·cm and 10−3 mole/L. Consequently, a plate with a thickness of 1.5 mm is required to generate an absorption of 97% of the incident light, but also plates thicker than 7.5 mm were fabricated and used (G. Baur et al., Appl. Phys. Lett., Vol. 31, pp. 4-6 (1977); W. Greubel et, al. Elektronik, pp. 55-56 (1977)). Hence, the large thickness of the plates increases the dimensions of the display devices also in the thickness direction. As a consequence, thin or, for instance, flexible displays cannot be manufactured. In addition, again according to G. Bauer et. al. (Appl. Phys. Lett., Vol. 31, pp. 4-6 (1977)) the plates act as waveguides for the fluorescent light and the light travels distances between 1 to 100 cm in the plate before it is coupled out. Consequently, the light output (brightness) is strongly reduced by the overlap of the absorption and emission spectrum of the fluorescent dyes. To minimize other light losses, plates have to be produced from highly transparent materials with perfect optical surfaces and perfectly reflecting layers at the narrow sides are required.
Unfortunately, the above improvements have failed to yield display devices which can be used in a large range of applications in an economical and satisfactory way, and the need continues to exist for more efficient displays of high brightness, high contrast, a wide viewing angle, and optional multiple colors.